1. Technical Field
The inventive concept relates to a semiconductor memory device and, more particularly, to a phase change memory device and a method of manufacturing the same.
2. Related Art
A phase change material has a different status of an amorphous state or a crystalline state depending on a temperature. The phase change material has a lower resistance in a crystalline state than in an amorphous state and has a regular atomic arrangement. The phase change material may be a chalcogenide (GST)-based material which is a compound comprised of germanium (Ge), antimony (Sb) and tellurium (Te).
A phase change random access memory (PCRAM) is a memory device which stores and reads information by using a status change property of the change material and has a fast operation speed and a high integration. The phase change material of the PCRAM is phase-changed by the Joule's heat which is applied through a bottom electrode contact (BEC) serving as a heater.
In particular, in a reset operation of the PCRAM which phase-changes the phase change material in an amorphous state, an enormous amount of current should be applied for a short time and an amount of reset current affects a life span, a sensing margin and shrinkage of the device.
To heighten the height of the BEC or to reduce the contact area between the BEC and the phase change material, it is suggested to reduce the reset current. In particular, the method for forming a spacer on an inner side wall of the BEC to reduce the contact area between the BEC and the phase change material layer is an effective method to minimize the volume that the phase change material is phase-changed to the crystalline state or an amorphous state, thereby improving the operation speed and reducing the reset current.
FIGS. 1a and 1b are sectional views illustrating a method of manufacturing a conventional phase change memory device. First, as shown in FIG. 1a, a semiconductor substrate 101 having a diode 105 as a switching device formed in a predetermined portion of a first interlayer insulating layer 103, is prepared. A metal silicide layer 107 is formed on the diode 105. Next, a second interlayer insulating layer 109 is formed on the resultant structure of the semiconductor substrate 101 and the BEC hole 111 is formed in the second interlayer insulating layer 109 to expose the metal silicide layer 107.
Next, as shown in FIG. 1b, an insulating layer is formed on the resultant structure of the semiconductor substrate 101 including the BEC hole 111 and then etched back to form an insulating spacer 113 on a side wall of the BEC hole 111.
The insulating spacer 113 is typically formed of a nitride material. In a spacer etching process, the insulating spacer 113 formed of a nitride material is etched faster in an upper potion of the BEC hole 111 than in the bottom portion of the BEC hole 111. Therefore, the diameter of the upper portion of the BEC hole 111 may be different from that of the bottom portion of the BEC hole 111 due to the insulating spacer 113. Although the insulating spacer 113 is formed to reduce the contact area between the BEC and the phase change material layer, since the upper portion 115 of the insulating spacer 113 is over etched due to the etch rate difference, the diameter in the upper portion of the BEC hole can not be reduced and therefore it can not obtain the desired objective to reduce the reset current.
Meanwhile, the BEC serving as a heater should fast radiate the heat applied in the reset operation.
FIG. 2 is a diagram explaining the heat radiation efficiency of the phase change memory device. Referring to FIG. 2, the phase change material layer 205 is phase-changed by the heating of the BEC 203 and then the applied heat is radiated through the phase change material layer 205 (A) or through the bottom (B) or the side (C) of the BEC 203. The reference numeral 201 designates an interlayer insulating layer.
The heat radiating through the phase change material layer 205 is 3 to 18% of the heat applied to the BEC 205 and the heat radiating through the bottom of the BEC 203 is 60 to 72% of the applied heat. The heat radiating through the side wall of the BEC 203 is 21 to 25% of the applied heat. Accordingly, if the insulating spacer is formed in the BEC hole, approximately one-fourth of the heat applied to the BEC 203 should be radiated to the insulating spacer. However, the nitride material of the insulating spacer is inefficient to radiate the heat because of the nature of the material. Furthermore, as the thickness of the insulating spacer is increased to reduce the diameter of the BEC, the speed of the heat radiation becomes further delayed such that the operation speed of the device is lowered.